The magnesium oxide based flue gas desulfurization techniques have been gradually promoted and applied in the market of the coal fired boiler flue gas treatment, the market share of which accounts for less than 1% in 2005 and at present over 6%, and the wet magnesium oxide based flue gas desulfurization techniques have been recognized by more and more people. The magnesium oxide based desulfurization process is adopted and the desulfurization waste liquid is made into the magnesium sulfite product, which solves the disposal problem of the desulfurized gypsum produced from the traditional calcium based desulfurization, and the magnesium sulfite product can also be processed and made into magnesium sulfate by-product through later aerated oxidation and processing by equipment such as the three-effect evaporator, the sales revenue of the by-product can offset part of operation and maintenance costs of the desulfurization equipment, which has great market application advantage from aspects of both techniques and economics than the traditional calcium based manner.
The techniques of magnesium sulfate production through crystallization of the waste liquid of the traditional magnesium oxide based desulfurization of the coal fired boiler flue gas adopt the oxidation fan as an equipment for necessary oxidation reaction of the transformation from magnesium sulfite to magnesium sulfate, and steam is used as the medium of the evaporation, crystallization and drying, which therefore causes that the manner of producing magnesium sulfate through the desulfurization waste liquid needs a larger consumption of oxidation fans and steam, greatly increasing the operation costs of desulfurization. Moreover, in the process of aerated oxidation with the oxidation fan inside or outside the tower, the transformation and production efficiency from magnesium sulfite to magnesium sulfate is low, and the conversion rate is not high, which in some extent limits the application of the techniques of producing magnesium sulfate through the waste liquid after magnesium based desulfurization in the field of the coal fired boiler flue gas desulfurization. Chinese patent application publication No. CN1775681A provides a manner: blasting air into the tower and aerating the desulfurized seriflux in the tower to transform magnesium sulfite into magnesium sulfate, and then evaporating and concentrating in the three-effect evaporator. This forced aeration and oxidization process needs high-power oxidation fans and a large consumption of electric power.
Still some other processes produce magnesium sulfate or other magnesium compound through a process of forced oxidation, two-effect or three-effect evaporation and crystallization after the desulfurization, e.g. Chinese patent application publication No. CN1544331A provides a manner: forcedly oxidizing the waste liquid after desulfurization, adding a magnesium hydroxide emulsion to neutralize sulfuric acid in the waste liquid, and then generating a magnesium compound through evaporation and crystallization. Such evaporation and crystallization process consumes lots of resources, and the cost is high. The forced oxidation procedure needs to be operated by an aeration system, which cannot guarantee that the effective ingredients in the slurry are conversed to produce a mixed liquid in which the concentration of magnesium sulfate and magnesium sulfite is over 80% under the influence of aeration time, amount of oxygen supplied, temperature and the like. From the point of view of the actual project results at present, most of the projects finally return to disposing the desulfurization by-product through abandonment after operating for a period of time.